The invention is in the field of radio frequency anechoic chambers used to test the electromagnetic susceptibility of, and the emissions from, electronic equipment.
All electronic equipment capable of emitting radio frequency radiation must be tested for RF emission levels before being cleared for production and distribution in the United States. The sheer quantity of such equipment, the limited bandwidth available for communication carrier frequencies and population density make it necessary for electronic devices to be made to operate electrically "quietly" in such a manner as not to interfere with communications and controls.
To determine the level of unwanted emissions in equipment, a specimen computer or microwave oven, for example, is enclosed with a broad-band receiving antenna in an anechoic chamber covered with metal to neutralize ambient radiation and lined with absorber material, and the amplitude of emitted radiation over the test band is measured with the test piece in operation.
The second type of test that requires an anechoic chamber is the RF susceptibility test. The purpose of the test is to ensure that electronic equipment can remain functional when irradiated by high levels of RF energy. Military aircraft offer the most significant example of the need for this type of test. The radiation intensity of nuclear explosions has spurred the conversion of the wiring harnesses of fighters and bombers to fiber optic cables, which are immune to radio interference, but the actual circuits with components cannot be made completely transparent to RF frequencies. Automobile controllers are also subject to radiation-caused malfunction, and are tested for RF susceptibility at transmission amplitudes of 200 volts across a wide frequency band to ensure their survivability under electromagnetic stress. Susceptibility testing does not require receiving and measuring radiation amplitude with an antenna, since the amplitude of the transmitted wave is known from gages on the wave generating equipment.
In both the emission and the susceptibility test it is important that the radiation propagating between the source antenna and test piece be uniform and not altered in magnitude from one point to another. To achieve this, and to eliminate outside random radiation that would skew the test results, the tests are conducted in an anechoic chambers designed to approximate as closely as possible the ideal situation in which no ambient radiation is admitted into the chamber and all radiation purposefully generated in the chamber uniformly illuminates the item under test.
The chambers used for this purpose were originally square box-shaped rooms lined on the inside with RF absorbing material to minimize reflections, with the antenna and test piece being located at opposite ends of the room. The rooms were square rather than elongated or rectangular, until a dozen years ago when the original square shape gave way to a flared chamber with a central outward bulge between the test piece and the antenna housed in the respective opposite ends. The flared chamber represented a substantial advance in the state-of-the-art in chamber design, as is detailed in the original patent on a chamber of this shape, U.S. Pat. No. 4,507,660 issued on Mar. 6, 1985 to a colleague of the instant inventor, Leland H. Hemming.
The flared shape enabled the chamber to be narrowed, considerably reducing the construction cost due to the savings on the absorber material required to line the reduced wall area. Without the flare, a square chamber is superior to an elongated one, since the high level of reflection of the low-angle incident radiation in an elongated rectangular chamber destroys the uniformity of the incident waveform. The innovation of the mated-horn style enabled the economic advantage of the elongated shape to be enjoyed without the low-angle reflections, since the flare meets most incoming radiation head-on, at the most absorptive angles.
As discussed in the referenced patent, anechoic chamber design is completely driven by cost. These chambers are on the order of 80 feet long, 60 feet wide and 40 feet high and cost hundreds of thousands of dollars, with the price tag being a linear function of the wall area covered with the expensive RF absorber material. A cost-reducing design, therefore, is not in the same league as, a good buy on paint for the boardroom at a fire sale. Improvements are constantly being sought to alter the algorithms that define the tradeoff between cost and performance. Even a minor improvement enabling substitution of a slightly smaller chamber could save ten thousand dollars per chamber.
As chamber design driven by this cost imperative has undergone improvements, a performance limitation inherent in the flared design was discovered. Namely, energy that is radiated omnidirectionally from the test antenna diffracts when the wavefront breaks across the edge transitions between the flared central section and the box-shaped end terminations. This diffracted energy disturbs the uniformity of the electromagnetic field illuminating the test object, thus causing errors in the measured emissions, or distortion in the test results of the device under test for radiated susceptibility.
With test radiation wavelengths reaching 30 feet and longer for the worst case low range frequency of 30 Mhz., the aperture framed by the transition discontinuities seen by the wavefront is of the same order of magnitude as the wave itself. This is the parameter needed for reinforcement of diffracted electromagnetic waves, which will cause classic alternating amplitude maxima and minima spaced according to the wavelength of the illuminating energy and the distance of the source, expressed by the equation, lambda=s sin theta, "s" being the spacing between slits, or approximately the width of the aperture, and Theta being the angle of deviation of the diffracted energy from the incident wave.
Correcting this problem is critical to utilizing an anechoic chamber as has been demonstrated. If the advantage of the flared horn chamber can be retained and the disadvantage of the diffracted wave eliminated or substantially reduced, the resulting chamber could replace an even larger equivalent rectangular or square chamber than it does currently.